1. Field of the Invention
This invention resides in the fields of photovoltaic cells and quantum dots.
2. Description of the Prior Art
Solar cells that generate electricity through the photovoltaic effect require a combination of low cost and high efficiency in order for such cells to offer a practical alternative to traditional means of power generation. One way in which the cost of manufacturing a photovoltaic cell can be lowered is by the use of solution processing to form the layer of light-harvesting material that is part of the cell. The efficiency of the cell, however, depends on the cell materials, including the light-harvesting material. The optimal light-harvesting material is one that achieves a high short-circuit current density Jsc by maximizing the absorption of the sun's rays in both the visible and infrared spectra, and that one extracts a high level of work, in the form of a high open-circuit voltage Voc and a high fill factor FF, from each absorbed photon. For an input solar intensity Psolar (typically 100 mW cm−2), the power conversion efficiency η is defined as
  η  =                    V        oc            ⁢              J        sc            ⁢      FF              P      solar      
It has been reported by Sargent, E., in “Infrared photovoltaics made by solution processing,” Nat. Photonics 3, 325-331 (2009), and Hillhouse H. S., et al., in “Solar cells from colloidal nanocrystals: Fundamentals, materials, devices, and economics” Curr. Opin. Colloid Interface Sci. 14, 245-259 (2009), that the use of colloidal quantum dots as the light-harvesting material provides photovoltaic cells with high power conversion efficiencies. Colloidal quantum dot photovoltaics offer both the ability to form the light-harvesting layer by solution processing and the ability to tune the bandgap over a wide range, benefits that are available in both single-junction and multijunction cells. The ability to tune the bandgap also enables the use of inexpensive, abundant ultralow-bandgap semiconductors that are otherwise unsuitable for photovoltaic energy conversion. By combining lead chalcogenide quantum dots and Schottky junctions, photovoltaic cells with efficiencies of 3.4% have been achieved, as reported by Ma, W., et al., “Photovoltaic devices employing ternary PbSxSe1-x Nanocrystals,” Nano Lett. 9, 1699-1703 (2009), and others. Significant progress has also been achieved by sensitizing nanoporous TiO2 electrodes with a thin layer of colloidal quantum dots, with power conversion efficiencies of 3.2%. See for example Fan, S., et al., “Highly efficient CdSe quantum-dot-sensitized TiO2 photoelectrodes for solar cell applications,” Electrochem. Commun. 11, 1337-1330 (2009).
Both colloidal quantum dots and Schottky devices pose certain limitations photovoltaic efficiencies, however. In Schottky devices, both the Voc and the FF values have fallen well below their potential, and in cells sensitized by colloidal quantum dots, the Jsc values are generally lower despite the increases in Voc and FF.